The invention relates to a process for the preparation of liquid, storage-stable isocyanate mixtures of low color number containing carbodiimide (CD) and/or uretonimine (UI) groups, the isocyanate mixtures obtainable by this process, the preparation of blends from these isocyanate mixtures with additional isocyanates, and to a process for the preparation of prepolymers containing isocyanate groups and of polyurethane plastics, and preferably polyurethane foams.
Isocyanate mixtures containing CD and/or UI groups can be prepared in a simple manner using the highly active catalysts from the phospholine series, and particularly the phospholine oxide series of catalysts. Such isocyanate mixtures are prepared by the processes as described in U.S. Pat. Nos. 2,853,473, 6,120,699 and EP-A-515 933.
The high catalytic activity of the phospholine catalysts, and specifically of the phospholine oxide catalysts, on the one hand is desirable in order to start up the carbodiimidization reaction under gentle temperature conditions. However, on the other hand, no process is known to date which ensures effective termination of the phospholine catalysis or phospholine oxide catalysis without limitation. The carbodiimidized isocyanates tend to after-react, i.e. they release gas as a result of evolution of CO2. This then leads to a build up of pressure, for example, in the storage tanks, and especially at higher temperatures.
There has been no lack of attempts to discover an effective means of terminating the phospholine catalysis. Various terminators are mentioned, for example, in the patent specifications DE-A-25 37 685, EP-A-515 933, EP-A-609 698 and U.S. Pat. No. 6,120,699. These terminators include, for example, acids, acid chlorides, chloroformates, silylated acids and halides of the main group elements. The termination of the phospholine catalysts with acids, which, for example, can also be in the form of acid chlorides, is not sufficiently effective.
According to the teaching of EP-A-515 933, CD/UI-containing isocyanate mixtures prepared by means of phospholine catalysis are terminated with at least an equimolar amount, and preferably from 1 to 2 times the molar amount, based on the catalyst employed, of e.g. trimethylsilyl trifluoromethanesulfonate (TMST). In practice, however, it has been found that CD/UI-containing isocyanates prepared in such a way are of only limited suitability for the preparation of prepolymers, i.e. reaction products of these CD/UI-containing isocyanates with polyols. The correspondingly prepared reaction products of polyols and the CD/UI-modified isocyanates tend to release gas, which can lead to a build up of pressure in the transportation tanks or to foaming during the handling of such products.
This problem can be by-passed by employing the silylated acid to terminate the phospholine catalyst analogously to EP-A-515 933 in higher molar equivalents (e.g. 5: 1-10:1, based on the catalyst). In practice, however, it is then found that the resultant CD/UI-modified isocyanates have a significantly poorer color number. This then also applies to the prepolymers prepared therefrom.
This also applies if the phospholine catalyst is terminated with acids of the trifluoromethanesulfonic acid type, in accordance with U.S. Pat. No. 6,120,699. Prepolymers prepared from these CD/UI-modified isocyanates also have a considerably increased color number.
In the preparation of liquid, storage-stable isocyanate mixtures containing carbodiimide (CD) and/or uretonimine (UI) groups, significant variations are sometimes observed in the reactivity of the isocyanate employed, and therefore, in the reaction times required. An undesirable prolonging of the reaction time could be counteracted, for example, by increasing the reaction temperature and/or the catalyst concentration (and as a result the amount of terminator). However, this would be associated with process and/or safety risks and/or quality problems (such as, for example, increased color values).
Thus, the object of the present invention was to provide a simple and economical process for the preparation of liquid, storage-stable and light-colored isocyanate mixtures which contain carbodiimide and/or uretonimine groups that do not have the deficiencies referred to, and leads to liquid, storage-stable isocyanate mixtures of low color numbers.